As one application of a silicon structure, a planner light-wave circuit (PLC) is known. Among various optical devices for optical communication, the device using the planner light-wave circuit is superior in miniaturizing, functioning and integrating the device, because a process for manufacturing a semiconductor circuit can be used.
Usually, an optical waveguide circuit is manufactured by a PLC manufacturing technique as follows. At first, a lower clad layer is formed on a silicon substrate, and then a core film through which light is guided is formed. Here, the core film is patterned into a core of a desired layout through a photolithography and a dry etching by using photo mask. After that, a reflow layer film is formed to embed the core. Then, an upper clad layer is formed thereon. The optical waveguide circuit manufactured in this way can have various functions such as an AWG (Arrayed Waveguides Grating) for coupling/decoupling a plurality of lights of wavelengths, a VOA (Variable Optical Attenuator) for attenuating an input light by a desirable quantity, and an optical switch for outputting the input light to a desirable port, in accordance with the layout of the core.
The characteristics representing the optical features of the optical waveguide circuit are insertion loss (or propagation loss) and PDL (Polarization Dependent Loss). Usually, it is preferable that both of them are made as small as possible. Here, the PDL is a phenomenon in which the insertion loss of the optical waveguide is different in accordance with the polarization state of an input light, and this is represented by the separation amount between the insertion loss of a TE mode and the insertion loss of a TM mode. The separation is greatly caused by the double refraction of the optical waveguide. The double refraction implies that the optical waveguide has a different refraction index depending on a propagation direction of the light. The main factor of the generation of the double refraction is an internal stress of the optical waveguide (in other words, a stress acting to remain in equilibrium with a stress applied from outside).
As mentioned above, the optical waveguide is manufactured by laminating silicon oxide films on the substrate. At this time, usually, the process requires a heating process at a very high temperature such as about 1000° C. For this reason, when the substrate is made of silicon, a wafer is warped by the difference in thermal expansion coefficient between the deposited silicon oxide film and the substrate, and the manufactured optical waveguide receives a great stress from the substrate. This stress generates the PDL. For this reason, a technique is used in which various methods are considered and applied, to reduce the stress or provide the balance between the stresses so that the directionality does not appear.
As one approach among the methods, a structure is proposed in which the optical waveguide is separated from the silicon substrate. In this technique, the optical waveguide is partially floated from the silicon substrate in the shape of a bridge and the stress from the substrate is released to reduce the PDL. From now, such a structure is referred to as a bridge structure.
Hereinafter, a thermal optical phase shifter as an example of this bridge structure will be described, in which an effect other than the PDL reduction can be obtained from this structure.
In the optical waveguide structure, a method is often used in which a TO (Thermo-Optic) effect is used to add functions. This actively utilizes the physical phenomenon that a refraction index of glass material of the optical waveguide structure is changed dependent on temperature. For example, a Mach-Zehnder interferometer composed of a directional coupler in the optical waveguide circuit has a structure to branch an input light into two lights and re-combine them after the propagation of a distance equal to each other. Here, a metal heater is placed on the upper portion of one branched optical waveguide, and electric power is supplied to the heater. Consequently, the phase of the light to be guided can be changed by the generated heat. In such a configuration, when the branched optical waveguides are coupled to each other, the interference state is changed in accordance with a phase difference between the lights to be guided through the branched optical waveguides, so that an output light intensity can be changed. For example, when the phase difference is assumed to be a half of the wavelength of the light, the branched lights cancel off each other when they are coupled to each other. Thus, the output becomes approximately zero. Also, when the phase difference is assumed to be zero or equal to integer times of the wavelength, the light intensity can be extracted in its original state. Here, a portion of the circuit or structure for changing the phase is referred to as the thermal optical phase shifter.
These phenomena allow the Mach-Zehnder interferometer to function as an optical switch. Also, since the phase difference can be optionally given, it can be used as an optical attenuator through a continuous phase change.
In order to operate an optical device, the electric power is supplied to the heater, to control the refraction index. Thus, the consumed power necessary for the operation is desired to be small. Usually, the electric power required to shift the phase of the light having the wavelength of 1550 nm, which is often used in the optical communication, is about 400 mW without any change of the structure. If the waveguides for 40 channels are required, the electric power of about 16 W is required. Since this power is very large, it needs to reduce the electric power by modifying the optical waveguide structure.
As the typical approach for achieving the smaller consumed power of the thermal optical phase shifter in the planner light-wave circuit, the optical waveguide having the following structure is typically employed. That is, grooves are formed to put a portion of the optical waveguide functioning as the phase shifter between them. The groove is filled with air or is vacuumed. However, since a heat conductivity of gas is very low as compared with the silicon oxide film for the clad, the heat generated by the heater can be prevented from being diffused into the clad. Thus, the consumed power can be made very small. The above structure is referred to as a ridge structure, and the optical waveguide portion having such a structure is merely referred to as a ridge.
Usually, the thermal optical phase shifter having the above ridge structure is manufactured by the following way. After the formation of the optical waveguide, a metal film for the heater is formed on an upper clad film by a sputtering apparatus or an evaporating apparatus. Subsequently, the heater is formed in the desirable layout through a photolithography using a photo mask by a milling apparatus. Subsequently, similarly to a step of forming the core, the dry etching apparatus is used to form the grooves in the desirable positions. At this time, the heater is required to be sufficiently protected with a resist so that the heater is not damaged in the dry etching.
The following method is typically used in the ridge structure in order to improve a thermal efficiency.
(1) The lower clad layer of the optical waveguide is made thick. Consequently, a thermal resistance between the core and the silicon substrate is increased, to prevent the heat from diffusing into the silicon substrate.
(2) The width of the ridge is made narrow. In case of the ridge structure, the heat is mainly diffused from the ridge into the silicon substrate. Thus, as the width of the ridge is made narrower, the contact area between the ridge and the silicon substrate is made smaller, to suppress the thermal diffusion.
However, those methods have the respective limits because of the following reasons.
(1) As the lower clad layer is made thicker, the stress applied from the substrate to the silicon oxide film becomes stronger. Thus, the substrate is easy to be cracked. Also, when the double refraction is increased with the strong stress, the PDL of the optical waveguide is increased. Moreover, as the film becomes thicker, the manufacturing step time is longer.
(2) If the ridge width is made excessively thin, the guided light feels the roughness of a ridge side wall, and a dispersion loss is increased.
Even if the above measures are taken as much as possible, the effect of the smaller consumed power attained by the ridge structure usually is to reduce to an approximate half of the above consumed power less than the usual case. Thus, the value of 8 W for 40 channels cannot be said to be sufficiently small.
In this way, an approach proposed as the method of further reducing the consumed power in addition to the ridge structure is to employ a bridge structure. The bridge structure floats the optical waveguide in the shape of the bridge to separate the optical waveguide from the silicon substrate. In case of the thermal phase shifter having the ridge structure, the bridge structure is attained by separating the ridge from the silicon substrate. Similarly to the ridge structure to suppress the thermal diffusion to the clad, the bridge structure suppresses the thermal diffusion to the silicon substrate. Thus, the effect of the further great reduction in the consumed power is obtained. Originally, as mentioned above, the bridge structure has the feature of the reduction in the PDL, because the stress from the substrate that is caused due to a line expansion coefficient difference between the clad layer film and the substrate is released. Therefore, the employment of the bridge structure in the thermal optical phase shifter provides the two great effects of the smaller consumed power and the smaller PDL. Hereinafter, such a structure is referred to as the bridge structure, although being already described above. Also, the portion of waveguide floated in the shape of the bridge is referred to as a bridge.
The bridge structure can be attained by the technique referred to as a sacrifice layer etching that is well known as a MEMS (Micro Electro Mechanical Systems) technique. This technique is the method of depositing a sacrifice layer on a substrate in advance and finally etching only the sacrifice layer in an isotropic manner, as disclosed in Japanese Patent Application Publication (JP-P2004-37524A). This method is attained by using the silicon substrate and the sacrifice layer of PSG (PhosphoSilicate Glass, and performing the etching with hydrogen fluoride solution. Since the silicon oxide film has a high etching rate to the hydrogen fluoride solution when it is doped with phosphorous, it is possible to selectively etch only the PSG.
Although this method has a merit that a special apparatus is unnecessary, a seed film for the sacrifice layer must be formed between the lower clad layer ad the silicon substrate. One problem of this method is in that in addition to the increase in the film formation step, the silicon oxide film deposited on the sacrifice layer must be adjusted to function as the waveguide in the refraction index and the softening temperature while a high etching selection is held to the sacrifice layer. This implies that the design free degree of the entire manufacturing process is lost. Also, the silicon oxide film is etched more or less with hydrogen fluoride in addition to the sacrifice layer film. Thus, the design and technique for manufacturing without any damage as the optical waveguide are required.
From the above, there is a method of etching the silicon substrate as a substitution approach. In this case, since the sacrifice layer is unnecessary, the bridge structure can be attained without any influence on the conventionally established process for manufacturing the optical waveguide. The method is disclosed in “Bridge-Suspended Silica-Waveguide Thermo-Optic Phase Shifter and Its Application to Mach-Zehnder Type Optical Switch” by A. Sugita et al. (Trans. IEICE, Vol. E73 (1990) pp. 105-109).
Although disclosed in the above paper, the etching of the silicon substrate is usually attained such that after the formation of the ridge structure, the silicon substrate is immersed in etching solution or exposed to etching gas. Although it is described in the above paper that the silicon substrate is etched by the dry etching, the isotropic etching of the silicon substrate can be attained by using the gas such as xenon fluoride.
In conjunction with the above description, an optical circuit is disclosed in Japanese Patent Application Publication (JP-P2004-133130A). The optical circuit has a silicon substrate, an optical waveguide layer and a temperature controller. Here, the optical waveguide layer has a core and a clad, which are formed on the silicon substrate. The temperature controller is formed on the optical waveguide layer and controls the temperature of a part of the core locally and variably. The optical waveguide layer is removed in an interval from the core from the optical waveguide layer surface to the silicon substrate surface along the longitudinal direction of the core in both sides of the formation portion of the temperature controller. A rectangular concave section is formed in the silicon substrate surface portion in the entire region opposite to the lower portion of the removal portion of the optical waveguide layer. The surface of the silicon substrate has a (100) plane. The sidewall of the concave section formed along the longitudinal direction of the core has the (100) plane that is substantially perpendicular to the silicon substrate surface. The bottom plane of the concave section has the (100) plane that is substantially parallel to the silicon substrate surface.
Here, in the etching the silicon substrate as well as the sacrifice layer, there are two issues when the isotropic etching is carried out to manufacture the bridge structure.
At first, the above paper indicates that the silicon substrate immediately under the optical waveguide is removed by the isotropic dry etching. FIGS. 10A to 10C are diagrams showing states that the silicon substrate is partially removed through an isotropic dry etching. In case of the isotropic etching, the etching advances independently of the crystal plane orientation of the silicon substrate. Thus, the silicon substrate immediately under the optical waveguide is in one of the state that as shown in FIG. 10C, the silicon substrate is partially removed and there is no portion where the optical waveguide is perfectly separated from the silicon substrate; and the state that as shown in FIG. 10A, the whole silicon substrate is removed, and the optical waveguide is perfectly separated from the silicon substrate.
In the former case, the optical waveguide and the silicon substrate are not separated. Thus, the thermal diffusion is generated through the silicon substrate whose thermal conductivity is high. As a result, an object of reducing the consumed power cannot be achieved. Also, since the stress from the silicon substrate is not perfectly released, another object of reducing the PDL cannot also be achieved.
On the other hand, in the latter case, when the optical waveguide is perfectly separated from the silicon substrate, the thermal diffusion to the silicon substrate is greatly suppressed. However, since the optical waveguide is not supported by any supporting member, this structure is mechanically weak. When the region of the silicon substrate to be etched is narrow, the mechanical strength can be reinforced by forming a plurality of beams perpendicularly to the optical waveguide, as shown in FIGS. 10A to 10C. However, when the etching region is wide, there is not the member for supporting the optical waveguide from below. Thus, the optical waveguide is easily broken by an external force. Moreover, when there are the beams, the thermal diffusion is performed through the beam, and the effect of the reduction in the consumed power cannot be attained. In addition, when there is an interference structure represented by the Mach-Zehnder type, it is necessary to design the optical waveguide so as to consider the thermal diffusion in such a way that the heat added to the thermal optical phase shifter does not thermally interfere with the other optical waveguide.
From the above reasons, when the isotropic etching is performed to form the bridge structure, there is an approach to provide not the beam but a supporting column. For example, as shown in FIGS. 11A to 11C, the width of the ridge is made wide or an island is formed in the position of the supporting column to be provided in the optical waveguide. FIGS. 11A to 11C are shown in Japanese Patent Application Publication (JP-P2004-37524A). Here, a portion 12a of the silicon substrate 11 remaining immediately under the portion 24c wide in the ridge width, or an island portion 24c functions as the supporting column. For this purpose, a portion of the silicon substrate immediately under an optical waveguide 24 must be perfectly etched to separate the optical waveguide 24 from the silicon substrate 1. Moreover, the portion 12a of the silicon substrate 11 immediately under the wide portion or island portion 24c must not be entirely etched, and the etching must be stopped in the state that the optical waveguide 24 and the silicon substrate 11 are not separated.
However, in this case, there is the following issue. Clad layers 13 and 15 are left in their original states in the wide portion or island portion 24c without being etched. Thus, the thermal diffusion to the left clad layers 13 and 15 is generated to lower the thermal efficiency. Also, the design must be carried out in consideration of the influence of the thermal interference, and the degrees of freedom in the layout of the optical waveguide 24 are lost. For example, in order to leave a supporting column 12a of silicon having the same width as the ridge, the ridge width of the portion serving as the supporting column 12a must be doubled. Therefore, the design must be performed in consideration of the above matter in addition to structural and thermal interference.
The second issue in forming the bridge structure by using the isotropic etching will be described below. For example, in the above paper, the etching of the silicon substrate portion immediately under the optical waveguide advances at a same rate from both sides of the ridge. When the silicon substrate continues to be etched, the optical waveguide is separated from the silicon substrate, as shown in FIG. 10A. However, it should be noted that the optical waveguide is separated simultaneously and instantly in the entire portion. Usually, a high temperature treatment between about 800 and 1300° C. is performed on the films of the optical waveguide. Thus, the wafer is largely warped due to the thermal stress based on the difference in thermal expansion coefficient between the silicon oxide film of the waveguide and the silicon substrate. In short, the silicon oxide film receives the strong stress from the silicon substrate. Therefore, when the stress from the silicon substrate is instantly released, there is a case that the bridge is broken.
Actually, this issue is not limited to the isotropic etching. For example, in Japanese Patent Application Publications (JP-P2001-521180A and JP-P2004-133130A), a method is disclosed of removing a portion of a silicon substrate immediately under a straight optical waveguide by using an anisotropic etching. Since the anisotropic etching of the silicon substrate by using an etching solution does not require a special apparatus, the bridge structure can be easily formed. Also, a wet anisotropic etching solution is represented by potassium hydroxide (KOH), 4-methyl ammonium hydroxide (TMAH), ethylene diamine pyrocatechol (EDP) and the like. However, in particular, the KOH solution etches silicon to silicon oxide at the selection ratio of about several hundred times. Thus, the KOH solution has a feature that damage to the silicon oxide film of the optical waveguide can be largely reduced. However, the etching of the silicon substrate advances at the same rate from both sides of the optical waveguide even in any portion of the optical waveguide, in case of Japanese Patent Application Publication (JP-P2004-133130A). Thus, the separation of the optical waveguide from the silicon substrate is regarded as an instant phenomenon, which does not lead to the solution of the issue.
Naturally, it should be noted that the bridge structure in which the above-mentioned issues are settled cannot be applied only to the optical waveguide circuit, but also various fields other than the bridge structure through the etching the sacrifice layer, such as a cantilever in the MEMS field.